Lower attachment for trimmable horizontal stabiliser actuator

ABSTRACT

A lower attachment system for a trimmable horizontal stabiliser actuator (THSA) includes a screwshaft forming a part of or coupled to a main screw of the actuator, the screwshaft having an axial direction and a primary ballnut disposed on the screwshaft and forming a part of a primary load path of the lower attachment system. The THSA also includes a lower secondary nut and an upper secondary nut as well as a secondary connection arranged for axial and rotational movement and coupled to each of the lower secondary nut and the upper secondary nut. The secondary connection is arranged to be loaded when the secondary load path is loaded and the secondary load path also includes the upper and lower secondary nuts.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation of U.S. application Ser. No. 15/080,791, filed onMar. 25, 2016, which claims the benefit of EP Application No. 15305450.7filed Mar. 27, 2015, the disclosures of which are incorporated herein byreference.

FOREIGN PRIORITY

This application claims priority to European Patent Application No.15305450.7 filed 27 Mar. 2015, the entire contents of which isincorporated herein by reference.

TECHNICAL FIELD

The invention relates to a lower attachment for a flight actuator suchas a trimmable horizontal stabiliser actuator. In particular, it relatesto adaptations to elements for use in a secondary load path of the lowerattachment for an actuator that has a primary load path and a secondaryload path.

BACKGROUND OF THE INVENTION

It is well known for flight actuator to include two mechanical loadpaths, one primary and the other secondary, with the latter intended totake up the load when the primary path has failed. In a typical priorart device, as shown in FIG. 1, when operating on the primary load paththe loads are transmitted through a hollow ball or roller screw. Thehollow screw houses a safety rod, also called a failsafe bar or tie bar,which is connected to the screw with a small amount of play. Duringnormal operation of the screw, when the primary load path is workingcorrectly, the secondary load path formed by this tie bar carries noload since there is no contact due to the small amount of play. However,in the event of a failure of the screw in the primary load path then thetie bar performs its failsafe function and ensures continuity of thetransmission of loads by the actuator.

With reference to FIG. 1 a typical known flight control actuator of thetrimmable horizontal stabiliser actuator (THSA) type includes a primaryload path with a hollow screw 32 connected at its upper end to theaircraft via a Cardan joint system 24 joining with first aircraftstructural elements S1. The primary load path further includes a nutassembly 25 mounted on the screw 33, and the nut assembly 25 isconnected to the stabiliser 22 of the aircraft, this connection beingachieved for example by another Cardan joint system 26.

As mentioned above, the secondary load path is provided by means of atie bar 29 which is within the screw 32. The tie bar 29 is terminated atits upper end by a male portion, in this case taking the form of aspherical head 27, which is mounted within a female portion on afastening piece 28, in this case taking the form of a recess 210. Thefastening piece 28 is connected to the structure of the aircraft viasecond aircraft structural elements S2. The known system may alsoinclude some means for preventing motion of the nut assembly 25 relativeto the screw 32 and/or for fixing the stabiliser 22/Cardan joint 26 inplace when the primary load path fails. Thus, the lower attachment, ofwhich the nut assembly 25 is a part, could also include secondary loadpath elements used when the primary load path fails.

In the example known upper attachment shown in FIG. 1 it will beunderstood that should the screw 32 fail then load can be carried by thesecondary load path since movement of the spherical head 27 isrestricted by the upper and lower shoulders of the recess 210.Consequently, the stabiliser 22 can either be safely held in a singleposition (in the case where the lower attachment locks in place) or insome prior art arrangements it might be permitted to continue normalmovement should the tie bar 29 be connected with the ballscrew 23 insuch a way as to permit continued rotation of the screw 23 even afterfailure preventing it from carrying axial loads.

A flight actuator with the basic features discussed above can be foundin the prior art, for example in U.S. Pat. No. 8,702,034 and in US2013/105,623.

SUMMARY

Viewed from a first aspect the present invention provides a lowerattachment system for a trimmable horizontal stabiliser actuatorcomprising: a screwshaft forming a part of or coupled to a main screw ofthe actuator, the screwshaft having an axial direction; a primaryballnut disposed on the screwshaft and forming a part of a primary loadpath of the lower attachment system; a lower secondary nut disposed onthe screwshaft axially adjacent the primary ballnut; an upper secondarynut disposed on the screwshaft adjacent the lower secondary nut and onthe opposite side thereof to the primary ballnut and a secondaryconnection arranged for axial and rotational movement and coupled toeach of the lower secondary nut and the upper secondary nut; wherein thesecondary connection is arranged to be loaded when the secondary loadpath is loaded and the secondary load path also includes the upper andlower secondary nuts; and wherein the couplings between the secondaryconnection and the upper and lower secondary nuts are arranged such thatduring normal use, when the primary ballnut transmits the loading of theactuator along the primary load path, then there is no relativerotational movement of the secondary connection, the upper secondary nutand the lower secondary nut, and such that when the secondary connectionis subject to an axial load resulting from load being transferred to thesecondary load path the upper secondary nut and lower secondary nut arecompelled to rotate relative to one another thereby locking the threadsof the secondary nuts with the screwshaft.

With this arrangement when sufficient loading is placed on the secondaryload path, i.e. when the primary load path fails, then the lowerattachment will become locked to the screwshaft and further movement ofthe lower attachment is prevented due to the locked threads of the uppersecondary nut and lower secondary nut. By locking the lower attachmentit becomes possible to lock the entire actuator in place so that after afailure of the primary load path the aircraft can safely be landed.

When the upper secondary nut and the lower secondary nut move inrelative rotation then their threads will become locked with the threadsof the screwshaft. This is because the two nuts are restricted frommoving axially relative to one another. In fact preferably no relativeaxial movement is permitted. It will however be appreciated that onlyone out of the upper secondary nut and the lower secondary nut needs tomove in relative rotation compared to the secondary connection.Preferably, just one of the two nuts moves rotationally relative to thesecondary connection. In this case the other nut need not move at all,which can simplify the mechanism.

The system may include a biasing mechanism disposed between the ballnutand the lower secondary nut, the biasing mechanism being arranged tohold the lower secondary nut in such a position that its threads aredisposed centrally between the threads of the screwshaft when no axialload is applied to the lower secondary nut. In this case when the axialload arising from failure of the primary load path is applied to thesecondary connection, then the coupling between the secondary connectionand the lower secondary nut may transfer an axial load to the lowersecondary nut, this axial load acting against the force applied by thebiasing mechanism and hence moving the threads of the lower secondarynut so that its threads are no longer disposed centrally between thethreads of the screwshaft. In some examples the threads of the lowersecondary nut bear on the screwshaft threads when the axial load istransferred from the secondary connection.

The couplings between the secondary connection, the upper secondary nutand the lower secondary nut may be implemented in a number of ways andthe only essential feature is that the function described above isachieved. For example, one of the upper and lower secondary nuts may becoupled to the secondary connection by surface to surface contactgenerating friction. This might be used when the nut in question isintended to remain immovable axially and radially relative to thesecondary connection. One of the upper and lower secondary nuts may becoupled to the secondary connection by a mechanism that preventsrelative rotational movement when no axial load is applied to thesecondary connection, but permits relative rotational movement when anaxial load is applied to the secondary connection, in particular anaxial load above a threshold level.

In an example implementation for the couplings one of the secondarynuts, preferably the upper secondary nut, is coupled to the secondaryconnection via a piston constrained within a slot and held in positionin the slot by a ramp; wherein the slot is shaped such that when thepiston is able to pass the ramp then the nut and secondary connectioncan move rotationally relative to one another thereby locking thethreads of the two nuts with the screwshaft. The piston may be heldagainst the ramp by a resilient device, for example a spring, such thatsufficient rotational force will cause the piston to pass along theramp, depressing the resilient device and passing beyond the ramp andfurther along the slot to achieve the rotational movement describedabove.

The slot may be formed in the surface of one of the secondary connectionor the nut. The spring and optionally a part of the piston may be heldwithin an axial bore in the other of the secondary connection or thenut. Thus, the slot and the piston are on separate sides of an interfacebetween the secondary connection and the nut and the pin bridges theclearance so that it can interact mechanically with both the secondaryconnection and the nut. Preferably the interface has inner and outercircumferential surfaces of the secondary connection and the nut facingeach other with a clearance in between.

The slot may have a first portion extending in the axial direction ofthe screwshaft with a second portion joining at an angle to an end ofthe first portion, the second portion extending in a circumferentialdirection. The circumferential extent of the second portion allows thepin to move circumferentially in the slot and this results in relativerotational movement of the secondary connection and the nut. The axialextent of the first portion enables the spring to push the pin into theend of the slot, making it impossible for reverse rotational movement tooccur. The second portion may extend both circumferentially and axially,i.e. not perpendicular to the first portion, and preferably the angle isan obtuse angle, for example an angle between 90° and 135°. The ramp ispreferably formed on the second portion and acts to keep the pin in thesecond portion until a sufficient rotational force is applied, at whichpoint the pin will pass by the ramp and be free to move along theremainder of the second portion, to the angle and into the firstportion.

There may be a thrust bearing between the upper and lower secondarynuts. This enables the two nuts to bear against each other with largecompressive forces and yet still rotate relative to one another in orderto jam the threads. It also permits compressive and tensile forces to beapplied between the two secondary nuts in normal use without any adverseimpact on the operation of the device, since in that case there will beno relative rotation and the threads of the two nuts will remaincorrectly oriented for rotational motion within the thread of the screwshaft.

The secondary connection may include transfer plates arranged tointeract with transfer plates on the primary ballnut. This may be fortransferring loads during normal use with elements of the secondary loadpath rotating at the same time as the primary ballnut. This and otherelements of the lower attachment and actuator that are not discussedabove can be implemented in a similar manner to the features of knownactuators of this type.

Viewed from a second aspect, the present invention provides a method ofusing a lower attachment system for a trimmable horizontal stabiliseractuator to avoid backlash in a secondary load path, the lowerattachment comprising: a screwshaft forming a part of or coupled to amain screw of the actuator, the screwshaft having an axial direction; aprimary ballnut disposed on the screwshaft and forming a part of aprimary load path of the lower attachment system; a lower secondary nutdisposed on the screwshaft axially adjacent the primary ballnut; anupper secondary nut disposed on the screwshaft adjacent the lowersecondary nut and on the opposite side thereof to the primary ballnut;and a secondary connection coupled to each of the lower secondary nutand the upper secondary nut; the method comprising: arranging thesecondary connection for axial and rotational movement; arranging thesecondary connection to be loaded when the secondary load path isloaded, wherein the secondary load path also includes the upper andlower secondary nuts; arranging the couplings between the secondaryconnection and the upper and lower secondary nuts such that duringnormal use, when the primary ballnut transmits the loading of theactuator along the primary load path, then there is no relativerotational movement of the secondary connection, the upper secondary nutand the lower secondary nut; and, when the secondary connection issubject to an axial load resulting from load being transferred to thesecondary load path, rotating the upper secondary nut and lowersecondary nut relative to one another in order to lock the threads ofthe secondary nuts with the screwshaft.

The method may include providing and/or using any or all features asdiscussed above in connection with the first aspect and optionalfeatures thereof.

The method may comprise providing a biasing mechanism disposed betweenthe ballnut and the lower secondary nut, and using the biasing mechanismto hold the lower secondary nut in such a position that its threads aredisposed centrally between the threads of the screwshaft when no axialload is applied to the lower secondary nut.

The method may comprise using the coupling between the secondaryconnection and the lower secondary nut to transfer an axial load to thelower secondary nut when the axial load arising from failure of theprimary load path is applied to the secondary connection, this axialload acting against the force applied by the biasing mechanism andmoving the threads of the lower secondary nut so that its threads are nolonger disposed centrally between the threads of the screwshaft.

The method may comprise providing a coupling between one of thesecondary nuts, preferably the upper secondary nut, and the secondaryconnection via a piston constrained within a slot and held in positionin the slot by a ramp; wherein the slot is shaped such that when thepiston is able to pass the ramp then the nut and secondary connectioncan move rotationally relative to one another thereby locking thethreads of the two nuts with the screwshaft; preferably so that thepiston is held against the ramp by a resilient device, for example aspring, such that sufficient rotational force will cause the piston topass along the ramp, depressing the resilient device and passing beyondthe ramp and further along the slot to achieve the rotational movementdescribed above. The method may include using the coupling in the stepof rotating the upper secondary nut and lower secondary nut relative toone another in order to lock the threads thereof.

The method may comprise providing the slot in the surface of one of thesecondary connection or the nut; preferably so that the spring andoptionally a part of the piston may be held within an axial bore in theother of the secondary connection or the nut, so that the pin bridgesthe clearance and so that it can interact mechanically with both thesecondary connection and the nut.

The method may comprise forming the slot so that it has a first portionextending in the axial direction of the screwshaft with a second portionjoining at an angle to an end of the first portion, the second portionextending in a circumferential direction.

The method may comprise providing the ramp on the second portion of theslot so that it acts to keep the pin in the second portion until asufficient rotational force is applied, at which point the pin will passby the ramp and be free to move along the remainder of the secondportion, to the angle and into the first portion.

The method may comprise providing a thrust bearing between the upper andlower secondary nuts and using the thrust bearing to enable the two nutsto bear against each other with large compressive forces and yet stillrotate relative to one another in order to jam the threads.

The method may comprise providing the secondary connection with transferplates and using the transfer plates to interact with transfer plates onthe primary ballnut, for transferring loads during normal use withelements of the secondary load path rotating at the same time as theprimary ballnut.

BRIEF DESCRIPTION OF DRAWINGS

An exemplary embodiment of the present invention will now be describedby way of example only and with reference to the accompanying drawingsin which:

FIG. 1 discloses a prior art flight actuator;

FIG. 2 illustrates a typical arrangement for primary and secondary loadpaths in a typical horizontal stabiliser actuator (THSA);

FIG. 3 shows a schematic diagram for a lower attachment for a THSAillustrating the principles of a backlash cancellation system; and

FIG. 4 shows a magnification of part of the lower attachment;

FIG. 5 shows a schematic diagram for the deployment of the backlashcancellation system of the lower attachment; and

FIG. 6 shows a schematic of the steps involved in the deployment of thebacklash cancellation device.

DETAILED DESCRIPTION OF THE INVENTION

One possible arrangement for primary and secondary load paths in a THSAis shown in FIG. 2. A horizontal stabiliser 22 is connected bystabiliser arms and bushings to a nut assembly 25 which connects it to ascrew 23. In this case the nut assembly 25 includes a gimbal assemblywith primary gimble trunnions and a gimbal ring connected to primaryball nut trunnions and a ball nut body, which is coupled via balls tothe screw shaft of the screw 23. The nut assembly 25 forms the lowerattachment of the actuator in the primary load path. At the upper end ofthe actuator a joint system such as a Cardan joint system 24 willtypically include no back elements along with the primary load pathgimbal, which is coupled to the aircraft structure. During normal use,without any failure, the loading for the actuator is carried via theprimary path.

In the event of a failure of the primary path the loading is transferredto the secondary path. In this example at the lower attachment thehorizontal stabiliser 22 is connected by the stabiliser arms and thebushings to a secondary load path lower attachment 35 comprisingfailsafe plates and transfer plates coupled via secondary nut trunnionsand a secondary nut body to secondary nut threads which are joined tothe thread of the screw 23 when the secondary load path is engaged.Typically the secondary nut threads would be arranged to lock with thethreads of the screw 23 preventing movement of the horizontal stabiliser22 when the secondary load path is engaged. From the lower attachment 35the load in the secondary load path is transmitted via the screw 23along the tie bar tube/rod 29 through the male end of the tie bar rod,which in this example is a tie bar sphere 27, and to the secondary loadpath upper attachment 20.

In this situation it will be understood that the horizontal stabilisercan be rigidly connected to the screw shaft 23 and through the tie bartube 29 to the tie bar sphere 27 which is held by the upper attachment20, and that therefore aerodynamic loading applied to the horizontalstabiliser during flight will result in dynamic loading on theconnection between the tie bar sphere and the secondary upper attachment20. It has been realised that this can result in undesirable “flutter”potentially causing degradation in aircraft performance and even a lossof control for the pilot. Consequently, an upper attachment 20 isproposed that incorporates a system for cancellation of the backlashthat is necessarily present when the secondary load path is unloaded.

FIG. 3 shows a lower attachment of a trimmable horizontal stabiliseractuator (THSA) with a backlash cancellation system. The lowerattachment comprises an upper secondary nut 10, a lower secondary nut70, a secondary connection 30, a transfer plate 110, a failsafe plate130, a ballnut 90, a spring 80 and a thrust bearing 60. The uppersecondary nut 10 is equipped with an anti-rotation device comprising apiston 40 that is actuated by a spring 50 and which moves constrained bya slot 42 defined in the surface of the secondary connection 30, and anaxial bore 14 within the upper secondary nut 10. The movement of thepiston 40 is discussed in more detail below with reference to FIG. 4.

The upper secondary nut 10 features on its outer diameter a ball track21 so that it can rotate under axial loads. The ball track 21 isdisposed between the upper secondary nut 10 and the secondary connection30 to allow relative rotation therebetween. The internal threads 12 ofthe upper secondary nut 10 cooperate with the threads 202 of thescrewshaft 200. The upper secondary nut 10 is therefore disposed betweenthe screwshaft 200 and the secondary connection 30.

The upper secondary nut 10 is also positioned above the lower secondarynut 70 (in relation to the orientation of FIG. 3). The threads 72 oflower secondary nut 70 also cooperate with threads 202 of the screwshaft200. The lower secondary nut 70 is also disposed between the secondaryconnection 30 and the screwshaft 200. The thrust bearing 60 is disposedbetween the upper secondary nut 10 and the lower secondary nut 70, so asto enable rotation therebetween even when the system is undersignificant axial loads.

The ballnut 90 is disposed below the lower secondary nut 70 andcooperates with the threads 202 of the screwshaft 200 to enclose theball bearings 101. Therefore, rotation of the screwshaft 200 controlsthe position of the ballnut 90 relative to the screwshaft.

The spring 80 is disposed between the ballnut 90 and the lower secondarynut 70 and serves to bias a flange portion 74 of the lower secondary nut70 against a fuse shoulder 92 of the ballnut 90, as seen in FIG. 3. Thelower secondary nut 70 is therefore held in its normal operatingposition such that its threads 72 are centred between the threads 202 ofthe screwshaft 200.

The secondary connection 30 extends from adjacent the upper secondarynut 10 downwards, past the lower secondary nut 70, to support thetransfer plate 110, which may contact the transfer plate 120 in order totransfer loads.

As noted above, the upper secondary nut 10 also includes ananti-rotation device that comprises a piston 40 actuated by a spring 50.During normal operation when the secondary load path is unloaded, thisdevice prevents rotation of the upper secondary nut 10 relative to thesecondary connection 30. This is because the piston 40 includes a pinextending between the upper secondary nut 10 and the secondaryconnection 30, and because the movement of the piston 40 is restrainedby the shape of the slot 42. The piston 40 is attached to the spring 50which is located within a vertical bore 14 (see FIG. 4) defined in theupper secondary nut 10. As a result the piston 40 can only move axiallyrelative to the upper secondary nut 10 and is moveable between a lowerand an upper position.

FIG. 4 shows a front-on view (rotated 90 degrees with respect to FIG. 3)of the surface of the secondary connection 30, wherein the end of thepin part of the piston 40 is seen to be located at the bottom of a slot42 defined in the secondary connection 30. The dashed lines show thelower part of the slit 14 defined in the upper secondary nut 10(disposed behind secondary connection 30 in this image), where thespring 50 is located. The spring 50 thus biases the piston 40 in anupward direction, but the piston 40 is prevented from moving laterally(during normal operation) by a ramp 44 defined in the slot 42 and theupward urging of the spring 50. The piston 40 therefore maintains itslower position in the slot 42.

The configuration shown in FIG. 4 is that of normal operation when thelower attachment is not locked, since the piston 40 is located in itslower position. The slot 42 in the secondary connection 30 is shapedsuch that once the backlash cancellation system has deployed (i.e. whenthe piston 40 moves to its upper position) the spring 50 will hold thepiston 40 with the upper secondary nut 10 and other secondary load pathcomponents in a locked position.

When the primary load path is loaded and the secondary load path isunloaded, the disposition of the components is as shown in FIG. 3. Thethreads 12, 72 of the upper and lower secondary nuts 10, 70, are spacedequidistantly between the threads 202 of the screwshaft 200. Thesecondary nuts 10, 70 are therefore free to rotate with the ballnut 90.

However, in the event of primary load path failure and subsequentaerodynamic loading on the aircraft, the secondary connection 30 will bepulled downwards relative to the screwshaft. This will bear on the lowersecondary nut 70, causing the spring 80 to compress and the lowersecondary thread 70 to thus also move downwards relative to thescrewshaft 200. Then, the threads 27 of the lower secondary connection70 come into contact with the lower (tensile) sides of the threads 202of screwshaft 200.

Once this contact is made, the load from the secondary connection 30 ispartially transferred to the upper secondary nut 10, which is thereforealso drawn downwards, resulting in its threads 12 contacting the threads202 of the screwshaft 200. The screwshaft then applies a rotational loadon the upper secondary nut 10.

If the rotational load on the upper secondary nut 10 is sufficient, thepiston 40 located in the slot 42 will be urged over the ramp 44 (viaslight compression of the spring 50). The piston 40 will thus be freedto move up the incline of the slot 42, assisted by the expansion of thespring 50, and into its upper position. The resulting lateral movementof the piston 40 allows the upper secondary nut 10 to rotate relative tothe secondary connection 30. This relative rotation causes the threads12 of the upper secondary nut 10 to be urged against the upper side ofthe threads 202 of the screwshaft 200. This configuration is shown inFIG. 5.

The contact between the threads 12 of the upper secondary nut 10 and thethreads 202 of the screwshaft, in combination with the contact betweenthe threads 72 of the lower secondary nut 70, prevents further rotationof the secondary connection 30 relative to the screwshaft and thereforelocks the lower attachment in place.

In summary, and with reference to FIG. 6, the operation of the lowerattachment can be described in three stages:

when there is no load on the secondary connection 30, the threads 12, 72of the upper and lower secondary nuts 10, 70 are centred between theflanks of the primary screw threads 202 of the screwshaft 200;

when a load is applied to the secondary connection 30, it movesdownwards and the threads 12, 72 of both of the secondary nuts 10, 70press the flanks of the tensile side of the threads 202 of thescrewshaft 200;

the resulting rotational force applied to the upper secondary nut 10urges the piston 40 over the ramp 44 defined in the slot 42 of thesecondary connection 30, and the upper secondary nut 10 rotates relativeto the secondary connection 30 and urges its threads 12 into contactwith the compressive side of the threads 202 of the screwshaft 200.

Thus, once the lower attachment has been locked, any inversion of theload will not cause the upper secondary nut 10 to be driven back byvirtue of the location of the piston 40 in the locking position of theslot 42.

The presence of the thrust bearing 60 between the upper secondary nut 10and the lower secondary nut 70 allows the upper secondary nut 10 torotate—and hence the system to operate—even under significant axialloading.

The methods and systems described herein and shown in the drawingsprovide a lower attachment for a trimmable horizontal stabiliseractuator that improves reliability of operation and the cancellation ofbacklash. While the apparatus and methods herein have been shown anddescribed with reference to exemplary embodiments, those skilled in theart will appreciate that changes and/or modifications may be madethereto without departing from the scope of the present invention asdefined by the appended claims.

The invention claimed is:
 1. A lower attachment system for a trimmablehorizontal stabiliser actuator comprising: a screwshaft forming a partof or coupled to a main screw of the actuator, the screwshaft having anaxial direction; a primary ballnut disposed on the screwshaft andforming a part of a primary load path of the lower attachment system; alower secondary nut disposed on the screwshaft axially adjacent theprimary ballnut; an upper secondary nut disposed on the screwshaftadjacent the lower secondary nut and on the opposite side thereof to theprimary ballnut; and a secondary connection arranged for axial androtational movement and coupled to each of the lower secondary nut andthe upper secondary nut; wherein the secondary connection is arranged tobe loaded when the secondary load path is loaded and the secondary loadpath also includes the upper and lower secondary nuts; and wherein thecouplings between the secondary connection and the upper and lowersecondary nuts are arranged such that during normal use, when theprimary ballnut transmits the loading of the actuator along the primaryload path, then there is no relative rotational movement of thesecondary connection, the upper secondary nut and the lower secondarynut, and such that when the secondary connection is subject to an axialload resulting from load being transferred to the secondary load path,the upper secondary nut and lower secondary nut are compelled to rotaterelative to one another thereby locking the threads of the secondarynuts with the screwshaft; wherein the system further comprises a biasingmechanism disposed between the ballnut and the lower secondary nut;wherein the biasing mechanism is arranged to bias a portion of the lowersecondary nut against a shoulder of the primary ballnut.
 2. A lowerattachment system as claimed in claim 1, wherein one of the upper andlower secondary nuts is coupled to the secondary connection by amechanism that prevents relative rotational movement when no axial loadis applied to the secondary connection, but which permits relativerotational movement when an axial load above a threshold level isapplied to the secondary connection.
 3. A lower attachment system asclaimed in claim 1, wherein one of the secondary nuts is coupled to thesecondary connection via an anti-rotation device having a pinconstrained within a slot and that is held in position in the slot by aramp; and wherein the slot is shaped such that when the pin is able topass the ramp then the secondary nut and secondary connection can moverotationally relative to one another thereby locking the threads of thetwo nuts with the screwshaft.
 4. A lower attachment system as claimed inclaim 3, wherein the pin is held against the ramp by a resilient devicesuch that sufficient rotational force between the secondary nut andsecondary connection will cause the pin to pass along the ramp,depressing the resilient device and passing beyond the ramp and furtheralong the slot to achieve the relative rotational movement of the upperand lower secondary nuts.
 5. A lower attachment system as claimed inclaim 4, wherein the slot is formed in the surface of one of thesecondary connection or the nut; and wherein the resilient device isheld within an axial bore in the other of the secondary connection orthe secondary nut.
 6. A lower attachment system as claimed in claim 5,wherein the slot has a first portion extending in the axial direction ofthe screwshaft and a second portion joining at an angle to an end of thefirst portion, the second portion extending in a circumferentialdirection, and wherein the ramp is formed on the second portion and actsto keep the pin in the second portion until a sufficient rotationalforce is applied to cause the pin to pass by the ramp and to move alongthe remainder of the second portion, to the angle and into the firstportion.
 7. A lower attachment system as claimed in claim 1, whereinonly one of the upper secondary nut and the lower secondary nut movesrotationally relative to the secondary connection.
 8. A lower attachmentsystem as claimed in claim 1, wherein the biasing mechanism is arrangedto hold the lower secondary nut in such a position that its threads aredisposed centrally between the threads of the screwshaft when no axialload is applied to the lower secondary nut.
 9. A lower attachment systemas claimed in claim 8, wherein, when the axial load arising from failureof the primary load path is applied to the secondary connection, thecoupling between the secondary connection and the lower secondary nuttransfers an axial load to the lower secondary nut, this axial loadacting against the force applied by the biasing mechanism and hencemoving the threads of the lower secondary nut so that its threads are nolonger disposed centrally between the threads of the screwshaft suchthat the threads of the lower secondary nut bear on the screwshaftthreads when the axial load is transferred from the secondaryconnection.
 10. A lower attachment system as claimed in claim 8, thesystem comprising a thrust bearing between the upper and lower secondarynuts.
 11. A lower attachment system for a trimmable horizontalstabiliser actuator comprising: a screwshaft forming a part of orcoupled to a main screw of the actuator, the screwshaft having an axialdirection; a primary ballnut disposed on the screwshaft and forming apart of a primary load path of the lower attachment system; a lowersecondary nut disposed on the screwshaft axially adjacent the primaryballnut; an upper secondary nut disposed on the screwshaft adjacent thelower secondary nut and on the opposite side thereof to the primaryballnut; and a secondary connection arranged for axial and rotationalmovement and coupled to each of the lower secondary nut and the uppersecondary nut; wherein the secondary connection is arranged to be loadedwhen the secondary load path is loaded and the secondary load path alsoincludes the upper and lower secondary nuts; and wherein the couplingsbetween the secondary connection and the upper and lower secondary nutsare arranged such that during normal use, when the primary ballnuttransmits the loading of the actuator along the primary load path, thenthere is no relative rotational movement of the secondary connection,the upper secondary nut and the lower secondary nut, and such that whenthe secondary connection is subject to an axial load resulting from loadbeing transferred to the secondary load path, the upper secondary nutand lower secondary nut are compelled to rotate relative to one anotherthereby locking the threads of the secondary nuts with the screwshaft;wherein the system further comprises a biasing mechanism disposedbetween the ballnut and the lower secondary nut; wherein one of thesecondary nuts is coupled to the secondary connection via ananti-rotation device having a pin constrained within a slot and that isheld in position in the slot by a ramp; and wherein the slot is shapedsuch that when the pin is able to pass the ramp then the secondary nutand secondary connection can move rotationally relative to one anotherthereby locking the threads of the two nuts with the screwshaft.
 12. Alower attachment system as claimed in claim 11, wherein the pin is heldagainst the ramp by a resilient device such that sufficient rotationalforce between the secondary nut and secondary connection will cause thepin to pass along the ramp, depressing the resilient device and passingbeyond the ramp and further along the slot to achieve the relativerotational movement of the upper and lower secondary nuts.
 13. A lowerattachment system as claimed in claim 12, wherein the slot is formed inthe surface of one of the secondary connection or the nut; and whereinthe resilient device is held within an axial bore in the other of thesecondary connection or the secondary nut.
 14. A lower attachment systemas claimed in claim 13, wherein the slot has a first portion extendingin the axial direction of the screwshaft and a second portion joining atan angle to an end of the first portion, the second portion extending ina circumferential direction, and wherein the ramp is formed on thesecond portion and acts to keep the pin in the second portion until asufficient rotational force is applied to cause the pin to pass by theramp and to move along the remainder of the second portion, to the angleand into the first portion.